1. Field of the Invention
This invention relates to an improvement in chlorination stage bleaching in the process of chemical delignification of wood chips. More particularly, this invention relates to washing unbleached softwood pulp with alcohol which results in diminished production of chlorinated dioxins and furans during subsequent chlorination and alkaline extraction stages. The alcohol of choice is a mono-hydric alcohol such as methanol, ethanol, propanol, isopropanol, or octanol. The preferred alcohol is ethanol.
2. Description of the Prior Art
Four ingredients are necessary to make paper: (1) raw materials--such as wood from the forest; (2) energy--from coal, oil, gas, or wood by-products, e.g., bark or wastes from the paper making process itself; (3) water, much of which is used as a conveyer belt to transport material along the process and is recirculated, but some of which is discharged to the atmosphere as vapor from driers or as liquid after purification in a waste treatment plant; and (4) skilled operators and management.
Not all pulps are produced chemically. "Groundwood" or "mechanical pulp," as the names imply, is produced by grinding a log of wood against abrasive stone surfaces or between rotating steel discs with cutting bars against their faces to yield fibers and fragmented fiber bundles. Such pulp is used in newsprint and similar paper where high opacity and good printability are desirable properties but where mechanical strength is not a prime requirement.
Chemical pulping begins with cutting the wood into chips. The chips are screened, rejects being both oversize slivers or undersize fines, and are taken to the top of a "digester" or a high pressure cooking vessel. Chemicals are added and the reaction is allowed some time under a prescribed program of temperatures, for the lignin of the wood and some hemicelluloses to be dissolved and extracted from the chips. Then the cooked material is discharged discontinuously in a batch process or continuously into a blow-tank where steam and other volatiles are flashed off. The cooking liquor--which is now a "black liquor" because of the dissolved lignin--is passed on to a chemical recovery cycle.
The pulp is washed with water to remove black liquor on, for example, a series of wire covered rotating drums. The washed brown stock is screened, diluted, and may be passed on to arrays of centrifugal cyclonic cleaners to separate large and heavy "dirt"--e.g., silica or metal particles--before bleaching. Since the screening operation and cyclone cleaners are only efficient with dilute suspensions, while bleaching requires higher consistencies for economical reasons, the stock is "thickened" by extracting some of its water, using wire covered, perforated drums on which the stock is made to form a mat.
The thick brown stock is next subjected to a series of bleaching operations. These can vary widely both in the types of chemicals used and their sequences.
In a favored system called CEDED, (Chlorine-Extraction-chlorine Dioxide-Extraction-chlorine Dioxide) the pulp is first delignified with chlorine gas, then extracted with sodium hydroxide and finally bleached with chlorine dioxide. As is well known in the art, this first chlorination stage of bleaching often involves various combinations of chlorine and chlorine dioxide. Chlorine dioxide attacks lignin specifically to a far greater extent than it attacks cellulose--unlike chlorine which is a more indiscriminate oxidant--but it is more expensive. Thus, it is preferably used for the final steps. After the final bleaching, the bright stock is washed to leave the pulp mill and enter the paper mill.
Much concern has been expressed about the environmental effects of chlorinated compounds formed by bleaching chemical pulp. Although investigations are incomplete and debate continues as to whether these compounds represent any true risk to the environment, special attention has been given chlorinated dibenzo-p-dioxins and dibenzofurans. Results, previously obtained by analyzing sediments sampled outside a pulp mill, suggest that there exists a very close correlation between these groups of compounds as reported at the 7th International Symposium on Chlorinated Dioxins and Related Compounds (Dioxin '87) in Las Vegas, Nev. Studies have indicated that 2,3,7,8-tetrachlorodibenzofuran (2378-TCDF) can be used as an indicator of the presence of 2,3,7,8-tetrachlorodibenzodioxin (2378-TCDD), the corresponding chlorinated dioxin isomer, at lower levels.
Practical means for preventing or reducing formation of these and related compounds are being sought in laboratory and mill studies. Certain approaches, such as oxygen delignification or high substitution of chlorine dioxide for chlorine in the chlorination stage, involve great expense, both in terms of capital equipment and processing costs. In addition to cost, significant time is required to implement these options. Some inexpensive, shorter term solution is desired.
The prior art approaches to the reduction of TCDD/F levels have focused primarily on modification of the chlorination stage of bleaching. Swedish researchers, for example, have claimed that TCDD/F levels are exponentially related to the "chlorine multiple" or "Kappa factor"--actually saying that the critical factor is the amount of chlorine applied to a certain amount of lignin ("The Influence of Lignin Content and Bleaching Chemicals on the Formation of Chlorinated Dioxins, Dibenzofurans and Phenolics" by Axegard et al. and "Influence of Oxygen Pretreatment and Chlorine Ratio on the Formation of PCDDs and PCDFs in Pulp Bleaching" by Swanson et al., presented at the Dioxin '88 Conference in Umea, Sweden.). On the other hand, co-pending U.S. patent application Ser. No. 262,534 reports the discovery that the amount of chlorine actually may remain at conventional levels as long as the concentration of chlorine does not exceed a definite level at any time during the chlorination bleaching stage, as a method for controlling formation of these undesirable compounds.
Washing the pulp after chlorination with excess water lowered the levels of 2378-TCDD and 2378-TCDF in the subsequent E-stage by only 5-10%. This suggests that the potential is low for removing chlorinated dioxins and furans from bleached pulp by improved bleach plant washing.